1. Field of the Invention
The present invention generally relates to a wafer processing apparatus, and more particularly, to a wafer processing apparatus which holds a wafer on an electrostatic chuck such that the wafer is subjected to desired processing steps such as etching, ashing, film growth, sputtering, or doping.
2. Description of the Background Art
Electrostatic chuck techniques have recently come to be applied to many types of apparatus for subjecting a wafer to various processing steps; for example, a plasma etching apparatus and a film growth apparatus. With a wafer clamp which has conventionally been used (i.e., a clamp for holding the periphery of a wafer), impurities are likely to deposit on the periphery of the wafer, as well as the periphery of the wafer is unavailable for production. The electrostatic chuck is used for reasons of preventing deposition of impurities on the periphery of the wafer, rendering the outermost periphery of the wafer available for production, and resulting in improved product yield.
Further, use of the electrostatic chuck yields temperature uniformity which is more stable than that yielded by use of the wafer clamp. For this reason, the electrostatic chuck can ensure superior processing performance even when the diameter of the wafer is increased. Consequently, as the diameter of a wafer becomes larger, the electrostatic chuck technique will be more commonly employed for semiconductor manufacturing systems.
This electrostatic chuck involves many technical problems for practical use. For instance, high-temperature processing (at a temperature of more than 200xc2x0 C.) of a wafer has recently been carried out frequently. When a wafer is chucked by a heated electrostatic chuck during the high-temperature processing, the wafer may be warped and damaged by thermal stress. For this reason, to ensure stable operation of the electrostatic chuck, there has been employed a manner in which a wafer heating means is specially provided, and a pre-heated wafer is conveyed to a electrostatic chuck so as to be subjected to desired wafer processing.
FIG. 1 is a schematic diagram showing the configuration of an electrostatic chucking apparatus described in Japanese Patent Application Laid-open No. Hei4-288062. The chucking apparatus shown in FIG. 1 comprises a main chamber 10 and a sub-chamber 12. An electrostatic chuck 14 having a heater is disposed within the main chamber 10, and a resistance heating medium 16 is provided within the electrostatic chuck 14. A wafer 18 is processed on the electrostatic chuck 14 while being heated by the resistance heating medium 16.
A wafer support tool 20 is disposed within the sub-chamber 12. The resistance heating medium 16 is provided within the wafer support tool 20, as in the electrostatic chuck 14. The wafer 18 is pre-heated within the sub-chamber 12 by the resistance heating medium 16 before being subjected to high temperature within the main chamber 10.
Next, there will be given an explanation of how the wafer is damaged while being directly secured by a high-temperature electrostatic chuck.
FIGS. 2A and 2B show a wafer processing apparatus having a commonly-employed electrostatic chuck. More particularly, FIG. 2A is a cross-sectional view of a conventional wafer processing apparatus taken along a plane Axe2x80x94A shown in FIG. 2B, and FIG. 2B is a front view of the conventional wafer processing apparatus. The electrostatic chuck shown in FIGS. 2A and 2B is of a well known two-electrode type.
In FIGS. 2A and 2B, reference numeral 22 designates a processing chamber for shielding the interior thereof from outside air; 24 designates a dielectric plate which is provided within the processing chamber 22 for generating electrostatic force; 26 designates a first electrode placed in the dielectric plate 24; 28 designates a second electrode disposed concentrically with the first electrode 26 in the dielectric plate 24; 30 designates a first variable D.C. power supply provided in order to apply a predetermined D.C. voltage to the first electrode 26; 32 designates a second variable D.C. power supply provided in order to apply a predetermined D.C. voltage to the second electrode 28; 34 designates a wafer which is held on the surface of the dielectric plate 24 so as to be subjected to predetermined processing; 36 designates a heater provided for heating the wafer 34 to a predetermined temperature by way of the dielectric plate 24; and 38 designates a pusher which passes and receives the wafer to and from a transport robot (not shown) which is provided so as to convey the wafer 34 to the interior of the processing chamber 22 and place the wafer 34 on the surface of the dielectric plate 24. The elements located within the region designated by reference numeral 40 correspond to the structure of a conventional common electrostatic chuck of two-electrode type.
FIG. 3 is a flowchart for explaining holding operation of the electrostatic chuck provided in the conventional wafer processing apparatus.
As shown in FIG. 3, in step S1, the wafer 34 is transported to the interior of the processing chamber 22 from an unillustrated transport robot.
In step S2, the pusher 38 is raised to receive the wafer 34 from the transport robot. The wafer 34 that is transported into the processing chamber 22 is passed from the transport robot to the pusher 38 that has been raised to a predetermined elevated position.
In step S3, the transport robot retracts from the processing chamber 22. After retraction of the robot, the operation proceeds to step S4.
In step S4, the pusher 38 is lowered to place the wafer 34 on the dielectric plate 24.
In step S5, desired voltages (a pair of reverse voltages employed in ordinary cases) are supplied from the first and second variable D.C. power supplies 30 and 32 to the first and second electrodes 26 and 28 embedded in the dielectric plate 24. As a result, the wafer 34 is securely held on the dielectric plate 24 by an electrostatic force.
By reference to FIGS. 4A to 4C, there will be described development of a fracture in the wafer 34 resulting from the holding action of the electrostatic chuck. In FIGS. 4A to 4C, those elements which are the same as those shown in FIGS. 2A and 2B are assigned the same reference numerals, and repetition of their explanations is omitted here.
FIG. 4A shows the wafer 34 immediately after having been placed on the dielectric plate 24; FIG. 4B shows the state of the wafer 34 when it is heated; and FIG. 4C shows a fracture in the wafer 34 resulting from heating. In FIGS. 4A to 4C, arrows depicted by reference numeral 42 indicate the direction in which the wafer 34 expands upon being heated, arrows depicted by reference numeral 44 indicate the direction of the electrostatic attraction force between the wafer 34 and the dielectric plate 24, and the lengths of the arrows 44 indicate the magnitude of the electrostatic attraction force. Reference numeral 46 designates a warp arising in the wafer 34 during the course of a heating process, and reference numeral 48 designates a fracture in the wafer 34 resulting when the warp 46 becomes excessive.
The mechanism whereby the fracture 48 developing in the wafer 34 will now be described in detail. As shown in FIG. 4A, the wafer 34 placed on the dielectric plate 24 is secured on the same by application of a predetermined voltage to the first and second electrodes 26 and 28.
The wafer placed on the dielectric plate 24 extends in the radial direction, i.e., in the direction designated by arrows 42 shown in FIG. 4B due to thermal stress, as being heated by the heater 36.
However, the wafer 34 is fixedly held on the dielectric plate 24 by the electrostatic attraction force. Thus, expansion of the wafer 34 is hindered, thereby generating the warp 46 within the wafer 34. Particularly, in the case of the electrostatic chuck of two-electrode type, strong attraction force acts between the first electrode 26 and the second electrode 28, as indicated by arrows 44. In this case, warping stress greater than the attraction force acts around the center of the wafer 34, thereby causing noticeable deformation to arise in the center of the wafer 34.
When the energy stemming from the warp developed in the wafer 34 due to the thermal stress exceeds the amount of energy sufficient to break the wafer 34, the fracture 48 is developed in the wafer 34 as shown in FIG. 4C. In a case where the wafer 34 maintained at a normal temperature (about 40xc2x0 C.) is chucked on the dielectric plate 24 adjusted to a temperature of about 250xc2x0 C. by the heater 36, the fracture 48 through such a mechanism arises in about three seconds after the wafer 34 has been held on the dielectric plate 24. Such a fracture 48 arises in substantially the same manner in a case where a silicon wafer is used as the wafer 34 and in a case where a silicon wafer having an oxide film is used as the wafer 34.
As described above, under the conventional method in which a wafer is directory held on a high-temperature electrostatic chuck, warp arises within the wafer, and in the worst case the wafer is fractured. Thus, in addition to the main chamber, an apparatus for holding the wafer through use of the electrostatic chuck usually comprises a sub-chamber for pre-heating the wafer for the purpose of preventing development of such a fracture in the wafer.
However, such an apparatus comprises a plurality of chambers and hence becomes complicated in structure. Further, since the wafer that has been pre-heated in the sub-chamber must be conveyed to the main chamber, the temperature of the wafer drops during the course of transportation. Also, transportation of the wafer consumes time, thus deteriorating processing capability.
The present invention has been conceived to solve the previously-mentioned problems, and a general object of the present invention is to provide a novel and useful wafer processing apparatus.
A more specific object of the present invention is to provide a wafer processing apparatus capable of efficiently processing a wafer through use of an electrostatic chuck.
The above objects of the present invention are achieved by a wafer processing apparatus described as follows. The apparatus includes a mechanism for heating a wafer within a processing chamber. Also within the processor chamber is a dielectric plate on which the wafer is placed. At least two electrodes are embedded in the dielectric plate. Variable D.C. power supplies are provided and controlled by a computation instruction device, such as a controller, for supplying voltages to the respective electrodes. The apparatus further includes pre-heating means for pre-heating the wafer placed on the dielectric plate before the wafer is secured on and attracted to the same by application of the voltages.